Constant velocity joints are a special class of universal joint in which the angular velocity of the output and input shafts match identically, irrespective of the angle of the joint. Such CV joints find significant application in front wheel drive automobiles, and are usually applied two per driving axle and thus four per vehicle. There are a number of relatively expensive parts which go to make up a constant velocity joint, and different ones of the parts can wear in use. At least in part because they are now so widely applied, a rebuilding industry has grown up to refurbish or rebuild worn CV joints.
The rebuilding industry, to date, has used rather subjective judgment in determining, based on an examination of the parts disassembled from a joint to be refurbished, which parts are so worn as to require scrapping, which have additional life remaining, and which might be reused with minor repair or refinishing. If incorrect decisions are made, joints can be rebuilt with defective parts, bringing on another premature failure and rebuild. The alternate is, of course, to replace parts which need not be replaced, increasing the cost of the rebuilding operation.
The typical environment for rebuilding constant velocity joints is in the nature of an automobile repair shop or machine shop, and the mechanic or technician doing the rebuilding is expected to make subjective judgments on the degree of wear based on the appearance or feel of a defect. He attempts to gauge the depth of a groove or defect to subjectively determine whether the part in question should be scrapped, refurbished or reused. With the increasing popularity of front wheel drive cars providing more and more CV joints to be rebuilt, the prospect is for more and more mechanics and technicians, many with less and less experience, making these subjective judgments with good or bad results, and usually without any correlation between the judgment they are making and the actual impact of the defect on the operation of the CV joint. The detrimental economic impact is apparent in that environment; many parts which have remaining life will be scrapped, and many CV joints will be rebuilt with parts which should have been scrapped.